Band-spread tuning circuit for radio receivers



y 1944- F. CAPLIN ET AL. 2,354,104

BAND-SPREAD TUNING CIRCUIT FOR RADIO RECEIVERS Filed June 24, 1941 2sheets-sheet 1 F/lg. I

Fig.2

MAX. M/N

INVENTOR$ FRANK CAPL //V CHARLE Y/VE BY ATTORNEY July 18, 1944. F.CAPLIN ET AL 2,354,104

BAND-SPREAD TUNING CIRCUIT FOR RADIO RECEIVERS Filed June 24, 1941 2Sheets-Sheet 2 ARCH/MEDIAN 6CRN Fly {I INVENTORS FRANK CAPLl/V YCHARLESE. PAYNE B AT'TORNEY Patented July 18, 1944 BAND-SPREADTUNING CIRCUITFOR RADIO RECEIVERS Frank Caplin, Bournemouth, and Charles Edward Payne,Mitcham, England, assignors to Radio Corporation of America, New York,N. Y., a

corporation of New York Application June 24, 1941, Serial No. 399,436 gIn Great Britain July 9, 1940 14 Claims.

The present invention relates to tuning circuits for radio receivers andis more particularly concerned with tuning circuits employed forband-spreading purposes.

Band-spread tuning circuits are well-known but the majority suffer fromserious disadvantages due partly to the diillculty of combining thefeature of band-spreading with the feature of wide tuning range. Forinstance in the known arrangement where a small capacity "trimmer"condenser is connected in parallel with each tuning condenser, thedegree of band-spreading varies as the third power of the frequency andhence the degree of band-spreading is very different at the two ends ofthe tuning range. Band-spreading by means of an auxiliary variablecondenser in series with the main tuning condenser has also beenproposed but it is found that it is unsuitable where the total tuningrange or the tuning range for any one wave-band is wide.

It has also been proposed to band-spread by effecting small variationsin the tuning inductance but, while this method overcomes the abovedisadvantage, mechanical diiiiculties arise when it is employed in areceiver arranged to operate on a plurality of wavebands.

It is an object of the present invention to provide a tuning ,circuitemployed for band-spreading purposes which does not suffer from theabove disadvantage and which does not involve any complex mechanicalmechanism in its application.

According to the invention band-spreading is effected by a variableinductance which is not part of the tuning circuit proper but is causedto influence tuning by its effect on an inductance which is connected inparallel with the variable inductance and is adapted either to becoupled by a mutual-inductance coupling to the tuning coil or to beformed by part or all of the tuning coil itself.

With this arrangement the degree of bandspread is constant for eachwaveband and by employing a plurality of coupling circuits which may beswitched in as desired, the circuit is easily applicable to a receiverwhich is arranged to operate over a number of wavebands.

While the degree of band-spread is constant for each waveband, it varieswith frequency within any waveband and it is a further object of theinvention to provide an arrangement in which the degree of band-spreadis independent of frequency.

According to this feature of the invention the cuit, the resonantfrequency of which is below the lowest frequency in the waveband;

The invention will be better understood by the following descriptiontaken in conjunction with the accompanying drawings in which Fig. 1shows the tuning circuit in its simplest form;

Fig. 2 shows a curve giving the change 'in frequency over any wavebandwhen employing the circuit of Fig. 1;

Fig. 3 shows the circuit of Fig. 1 applied to a receiver having twotuned circuits;

Fig. 4 shows a second embodiment of the invention in its simplest form;

Fig. 5 shows a curve showing the same relation as the curve of Fig. 2but for the circuit of Fig. 4;

Fig. 6 shows the invention applied to the case where two wavebands areband-spread while Fig. '7 shows the invention applied to the case wheretwo tuned circuits are employed and bandspread tuning is employed overtwo wavebands;

Figs. 8, 9, l0 and 11 show four mechanical arrangements for effectingthe desired variation in the band-spread inductance.

Referring now to Fig. 1 of the drawings the tuning condenser isindicated by C and the tuning inductance by L1. The band-spread variableinductance L is coupled to the tuning circuit proper by means of theinductance L: in parallel therewith and mutually coupled to the tuninginductance L1. It will be seen that the current-in the variableinductance and consequently its effect on the main tuning inductance L1decreases with increasing frequency and thus the advantage of employinga variable inductance as against a variable capacity for band-spreadingis obtained and by employing a plurality of mutual inductances which maybe switched in as desired the arrangement is easily applicable to areceiver capable of receiving signals over a number of wavebands. Itwill also be appreciated that the invention may be applied to each of aplurality of tuned circuits, for instance diiferent stages in areceiver, by providing a plurality of matched variable inductancesganged together mechanically, one for each circuit and each associatedwith an appropriate coupling circuit and tuning coil as shown in Figure3.

It will be seen from the curve shown in Fig. 2 where band-spread a] isplotted against frequency for the circuit of Fig. 1 that the band-spreadchanges with frequency and is in fact proportional to the frequency. InFig. 2 the band-spread tuning range is from l min. to f max. This is adisadvantage from the point of view of convenience of operation andcalibra ion and can be avoided by employing the circuit shown in Fig. 4where a condenser C1 is inserted between the variable inductance L andthe coupling inductance Ls, the value of the condenser being such thatthe resonant frequency of the circuit L, 14, C1 is below the lowestfrequency in the waveband as shown in the curve of of Fig. 5. From thiscurve it will be seen that between the frequency 1' min. and f max., thedegree of band-spread has a substantially constant value. the deviationfrom this value at the centre of the band being quite small.

Fig. 8 shows a tuning arrangement for a receiver having two wavebands.Two coupling circuits formed of mutually coupled inductances Li, Ls andIn, L4 are provided and are arranged to be connected alternativelybetween the main tuning condenser C and the variable band-spreadinductance L by means of a two position switch having two switch arms 81and S2. If the ratio between the inductances Li. L: and In. Ll is thesame then the degree of band-spread will be constant over each waveband.Fig. 7 shows an arrangement for band-spread tuning over two wavebands ina receiver having two tuning circuits.

In Fig. 6, the condensers C1 and C: may be inserted if it is required torender the band-spread substantially independent of frequency over boththe wavebands as explained above.

In the arrangements shown in Figs. 8 and 10 in which similar referencesindicate similar parts the variable band-spread inductance L is wound ona suitable former III which is attached to a plate II which with the boxIl may form part of the screening arrangement of the receiver. A highfrequency iron core H is mounted on a spindle I! which is adapted to bemoved so that the core travels into or out of the inductance coil L. Aspring II is provided, one end of which is fixed to the plate ll whilethe other is fixed to the spindle I! so that the tendency is for thecore to be moved out of the inductance coil. The movement of the spindlemay be controlled as shown in Fig. 8 by an arrangement acting on theprinciple of the Archimedean screw. The spindle i2 is provided with astud It resting on curved surface I! of such a shape that on rotation bythe knob ll of the spindle II on which the surface I1 is fixed causes alongitudinal movement of the spindle If. It will be understood that bysuitably choosing the dimension of tha surface H a very fine movement ofthe spindle I: may be obtained. It will be understood that a pluralityof such units may be mounted on the spindle I! as shown in Fig. 9.

In Fig. 10 the movement of the spindle I2 is derived from a shaft 2'!arranged at right angles thereto. A cam 26 is mounted on th shaft 21 andthe cam engages with a roller 28 mounted on the spindle l2. Rotation ofthe shaft 21 thus causes the spindle I! to move longitudinally and thecore ii to move into or out of the inductance coil L. It will beappreciated that with this arrangement several cams may be mounted onthe shaft 21 so that ganged control of a number of stages of thereceiver is obtained as shown in Fig. 11.

We claim:

1. A band-spread tuning arrangement for radio receivers comprising amain tuning circuit having a tuning inductance, a variable band-spreadinductance separate from said tuning circuit, and means for couplingsaid variable band-spread inductance to said tuning inductance wherebyvariation of said variable band-spread inductance has the eifect ofvarying the tuning of the main tuning circuit due to the effect of saidbandspread inductance on said coupling means.

2. A band-spread tuning arrangement for radio receivers comprising amain tuning circuit, a variable band-spread inductance separate fromsaid tuning circuit, a second inductance connected in parallel with saidvariable band-spread inductance, and means for coupling said secondinductance to said tuning circuit whereby variation of said variableband-spread inductance has the effect of varying the tuning of the maintuning circuit due to the effect of said band-spread inductance on saidsecond inductance.

3. A band-spread tuning arrangement for radio receivers comprising amain tuning circuit, a band-spread inductance separate from said tuningcircuit, means comprising a movable core for varying said band-spreadinductance, and means for coupling said band-spread inductance to saidtuning circuit whereby variation of said variable band-spread inductancehas the eflect of varying the tuning of the main tuning circuit due tothe effect of said band-spread inductance on said coupling means.

4. A band-spread tuning arrangement as claimed in claim 3 wherein thesaid movable core is composed of high frequency iron.

5. A band-spread tuning arrangement for radio receivers arranged tooperate over a plurality of wave-bands and comprising a tuning circuit,a plurality of tuning inductances for said tuning circuit, a variableband-spread inductance, a plurality of coupling inductances for couplingsaid variable band-spread inductance to said tuning circuit, each ofsaid coupling inductances being mutually coupled to one of said tuninginductances, switch means for connecting said variable band-spreadinductance to any one of said coupling inductances and switch means forconnecting any one of said tuning inductances to said tuning circuit.

8. A band-spread tuning arrangement for radio receivers comprising a,main tuning circuit, a resonant circuit the resonant frequency of whichis below the lowest frequency in the waveband, a variable band-spreadinductance forming part of said resonant circuit, and a secondinductance connected in parallel with said variable bandspreadinductance and coupled to said tuning circuit.

7. A band-spread tuning arrangement for radio receivers arranged tooperate over a plurality of wavebands and comprising a tuning circuit, aplurality of tuning inductances for said tuning circuit, a plurality ofresonant circuits one for each wave-band and having a resonant frequencywhich is below the lowest frequency in the waveband with which it isassociated, a variable bandspread inductance forming a common part ofall said resonant circuits, a plurality of coupling inductances, each ofwhich is mutually coupled to one of said tuning inductances, switchmeans for connecting said variable band-spread inductance to any one ofsaid resonant circuits by way of one of said coupling inductances, andswitch means for connecting any one of said tuning inductances to saidtuning circuit.

8. A band-spread tuning arrangement for a radio receiver comprising atleast one tunable circuit provided with a fixed inductance and a shuntvariable tuning condenser for tuning the circuit over a comparativelywide frequency range, and an auxiliary circuit comprising a variableband-spread inductance and a shunt inductance which is mutually coupledto the fixed inductance of the tunable circuit, the arrangement beingsuch that adjustment of the bandspread inductance has the effect ofvarying the tuning of the tunable circuit over a comparatively narrowfrequency range.

9. A band-spread tuning arrangement for a radio receiver comprising atleast one tunable circuit provided with a fixed inductance and a shuntvariable tuning condenser'for tuning the circuit over a comparativelywide frequency range, an auxiliary circuit comprising a bandspreadmovable core inductance and a shunt inductance which is mutually coupledto the fixed inductance of the tunable circuit, and means for moving thecore to eil'ect adjustment of the bandspread inductance and coincidentlythe tuning of the tunable circuit over a comparatively narrow frequencyrange.

10. A band-spread tuning arrangement for a multi-band radio receivercomprising at least one tunable circuit for each band provided with afixed inductance and a shunt variable tuning condenser for tuning eachof said circuits over a comparatively wide frequency range in itsrespective band, and an auxiliary circuit for each band comprising avariable band-spread inductance and a shunt inductance which is mutuallycoupled to a corresponding fixed inductance of one Of the tunablecircuits, the arrangement being such that adjustment of the band-spreadinductance has the efi'ect of varying the tuning of each tunable circuitover a comparatively narrow frequency range in its respective band.

11. A band-spread tuning arrangement for a multi-band radio receivercomprising a fixed inductance for each band, a variable tuning condenserand means for selectively shunting said condenser across on. of thefixed inductances to form a circuit which is tunable over one of thebands. a second fixed inductance for each band mutually coupled to thecorrespondin fixed inductance of the tunable circuit, a variablebandspread inductance and means for selectively shunting saidband-spread inductance across one of the second fixed inductances.

12. A band-spread tuning arrangement in accordance with the inventiondefined in claim 11 wherein the selective shunting means are constitutedby switches which are controlled in unison.

13. A band-spread tuning arrangement for a multl-band radio receivercomprising a plurality of high frequency transformers corresponding tothe number of bands, 9. variable tuning condenser, means for selectivelyconnecting the tuning condenser in shunt across the primary of eithertransformer whereby the resulting circuit is tunable over acomparatively wide frequency range in its respective band, a variablebandspread inductance, means for selectively connecting said inductancein shunt across the secondary of either transformer whereby adjustmentof the band-spread inductance has the effect of varying the tuning ofeither tunable circuit over a comparatively narrow frequency range inits respective band.

14. A band-spread tuning arrangement in accordance with the inventiondefined in claim 13 wherein the selective connecting means areconstituted by switches which are controlled in unison.

FRANK CAPLIN. CHARLES EDWARD PAYNE.

